Solar Energy Perspectives - IEA

Chapter 4: Buildings
Pumps variant, by contrast, assumes the development of ultra-high efficiency air-conditioners
and faster cost reductions for space and water heating applications. In such case heat pumps,
which take most of their resource from the surrounding, renewable “ambient energy” would
become the dominant heating technology by 2050. This would still allow a significant role
for direct solar energy, as we shall see.
Heat pumps
A heat pump works in a similar way to a refrigerator. A refrigerator cools food by extracting
their heat, which is then released through a condenser. In the case of the heat pump for space
heating, the evaporator extracts heat from the environment (water, ground, outside air or
waste air) and adds this to the heating system through the condenser (Figure 4.7). In other
words, heat pumps extract heat from a relatively cold medium and lift its temperature level
before introducing it into a warmer environment. Apart from the electricity running the
pump, itself ultimately turned into heat, the origin of the energy is renewable – solar for airsource
heat pumps (ASHP) and “horizontal” ground-source heat pumps (GSHP) or surface
water-source heat pumps (surface WSHP), and a mix of solar and geothermal for “vertical”
GSHP or deep WSHP, depending on the depth at which they collect the heat.
Figure 4.7 How heat pumps work
Renewable
energy sources
Heat pump
Distribution system
Air
De-compression
Ground
Approximately
3/4
Evaporation
Condensation
4/4
Water
Compression
Heating
Cooling
Hot water
Approx.
1/4
Auxiliary energy (gas, electricity)
Notes: Whether used for cooling or for heating, heat pumps use a gas refrigerant, which a pump circulates between two heat
exchangers separated by a barrier (the wall of a house or of a refrigerator). In practice the heat exchangers are just hollow metal coils,
one is known as an evaporator and the other is known as a condenser. As the refrigerant enters the condenser it is compressed, which
raises its temperature. Then as it flows through the condenser it gives off this heat to its cooler surroundings. After the condenser, the
cooled but still pressurized refrigerant is allowed to expand as it reaches the evaporator. This drops its temperature to the point where
it is cool enough to absorb heat from its surroundings. The gas then returns to the condenser where the cycle repeats.
Source: EHPA/Alpha Innotec.
Key point
Heat pumps transfer heat from the cold outside to the warm inside.
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© OECD/IEA, 2011